The present invention relates to, but is not limited to, magnetoresistive (MR) heads that read magnetically encoded information. In particular, the present invention relates to a peak detection timing circuit modification which improves the error rate in recovered data, as a result of compensating for AC timing asymmetry in the timing signal which may be especially prevalent in MR heads.
Magnetoresistive heads are used to read back magnetically encoded information from thin films of magnetic media on rigid disc substrates. The error rate is a measure of the accuracy with which the encoded data is recovered and converted back to the original digital information. Head, preamplifier, media, and radiated interference noises, intersymbol (pattern dependent) interference, overwrite, and AC timing asymmetry are known loss mechanisms that contribute to a large error rate. Timing asymmetry (whether AC or DC) is often termed "pulse pairing".
AC timing asymmetry is defined for single frequency signals that are written over a much higher frequency signal (or over AC erased media). AC timing asymmetry is characterized by the timing difference that exists between each pulse and its expected location, which is exactly half way between its two neighboring pulses.
Constant (DC) magnetic fields from remanence in the magnetic write head, or from external sources, can cause AC timing asymmetry in any digital magnetic recording system, as can asymmetry in the write current waveform, which results from electronic offsets, non-linearities, or leakage current. Data recovery systems that derive timing information by performing a zero crossing comparison on the differentiated, filtered readback signal are susceptible to additional AC timing asymmetry as a result of a DC offset voltage at the input of this timing comparator.
Present digital magnetic recording systems that use inductive transducers to read signals are not particularly impacted by these sources of AC timing asymmetry because it is possible to mass produce systems in which all of these effects are small. Also, a high probability exists for some asymmetry effects to cancel others.
Signals are transduced with MR heads because the resistance of the MR element varies as a function of the angle between the magnetization vector in the element (M) and the electrical current flowing through the element (I). The MR element is designed to be magnetically permeable, so M is rotated, relative to its quiescent direction, by the unshielded fields that emanate from magnetic transitions in the moving media.
The fundamental MR effect has been characterized to have a cosine squared dependence on the angle between the I and M vectors. Therefore, the MR element resistance change is most rapid, and most linear, as M rotates through a 45 degree angle with respect to I (bias angle) and the MR element resistance change is very slow, and non-linear, as M approaches a zero or a 90 degree bias angle.
An MR element that operates around a quiescent bias angle of 45 degrees will have a symmetric and linear response to small rotations of M. The same element will have a very non-linear response to large M rotations, but will maintain a symmetric response. This symmetry is lost, especially for large rotations, as the quiescent bias angle shifts away from 45 degrees. The element then transduces a much more compressed pulse for one signal polarity as compared to the opposite polarity.
Transitions are written in thin longitudinal media by switching the polarity of the write field component that is applied along the axis of media motion (or at some relative skew angle). A transverse write field fringes from both sides of the write head and coincidentally writes transverse transitions. This is often referred to as "side writing".
To maximize signal-to-noise ratio, for a given system write-to-read mis-registration characterization, rigid disc applications use MR read element designs which read nearly as wide as the integral write heads write. Therefore, a summation of the fields from transverse, side written, transitions and from primary signal transitions are responsible for rotating M to produce a readback signal.
The resulting readback signal possesses a "baseline shift" characteristic, which varies as a function of write-to-read mis-registration. Baseline shift is most easily observed when reading back relatively isolated transitions. It appears as a band limited squared wave, which skews each pulse as it switches polarity approximately coincident with each pulse. It then holds the signal at a non-zero level, until the next pulse, of the opposite polarity, is encountered.
MR elements that are not biased symmetrically, as described above, will transduce signals in which the more compressed pulse is not affected as much (especially in terms of pulse skew) as the opposite (larger amplitude) pulse. This results in AC timing asymmetry, which varies as a function of write-to-read mis-registration. Accordingly, the preferred embodiment of the present invention is adaptive.
If either the I or the M vectors are not uniform across the element (barber pole biasing is a good example of non-uniform I) then a non-planar write gap will result in still another AC timing asymmetry term. A stepped write gap is common because the writer is deposited on top of the contour formed by the MR contacts, which are patterned to form the reading portion of the MR element. However, additional processing steps are capable of significantly diminishing the magnitude of these write gap steps. If a stepped write gap and non-uniform MR element biasing coexist, then two signals of different amplitude symmetry will be transduced at two slightly different times to form a composite waveform. This composite waveform will possess AC timing asymmetry that varies as a function of write-to-read mis-registration.
When reading magnetic signals with MR heads, significant additional AC timing asymmetry may exist, which has been observed to degrade error rate performance by as much as 2 decades (a factor of 100).